1. Field of the Invention
The present invention relates to a communication terminal, a mobile communication system, and a communication control method.
2. Description of the Related Art
Wireless communication systems for variably controlling downlink (from base station to terminal) communication speeds (hereinafter, referred to as “downlink speeds”) according to a reception state at each communication terminal are known, as typified by CDMA2000 1xEV-DO. These wireless communication systems employ a wireless communication technique for achieving a tradeoff between downlink speed, and, for example, error resilience by exploiting a plurality of modulation techniques, spread rates, etc., so that a higher communication speed can be provided according to the reception state at each communication terminal. A communication terminal based on this wireless communication technique measures the quality of a signal (the carrier to interference code (CIR) of the signal) received from the base station and requests the highest possible downlink speed for assuming successful data reception from the base station with error rates equal to or below a predetermined level. On the other hand, in response to requests about downlink speeds from a plurality of communication terminals, the base station performs scheduling of the requests and determines a communication terminal to which downlink communication information is to be transmitted. Although telecommunication carriers can employ any desired technique for this destination determination scheduling, a scheduling algorithm called proportional fair scheduling is commonly employed. This scheduling technique features a good balance between fairness of communication speeds among a plurality of subscribers and a maximized total throughput in the overall base station. More specifically, in the proportional fair scheduling, an average communication traffic volume R over a past period of certain length is calculated (typically, the average of traffic over the past one second or so is calculated based on the moving average or logarithmic mean) for each communication terminal to assign downlink communication information to a communication terminal exhibiting the maximum ratio of DRC to R (DRC/R), where DRC is a requested downlink speed and R is the above-described average communication traffic volume (refer to, for example, Japanese Unexamined Patent Application Publication, First Publication No. 2002-171287).
On the other hand, for a wireless communication system in which a base station communicates with communication terminals, there is a known communication technique for achieving communication between the base station and a communication terminal outside the communication area covered by the base station by using another communication terminal existing in the communication area of the base station as a relay station (refer to, for example, Japanese Unexamined Patent Application Publication, First Publication No. 2003-309512).
Also, for a wireless communication system in which a base station controls the transmission power of each communication terminal according to the reception state at the communication terminal, there is another known technique for preventing overheating due to heat generated in a transmission power amplifier by decreasing the transmission output when the temperature exceeds a preset permissible temperature and increasing the transmission output to the original level when the temperature falls below the permissible temperature (refer to, for example, Japanese Unexamined Patent Application Publication, First Publication No. 2000-083009).
In the wireless communication system described in Japanese Unexamined Patent Application Publication, First Publication No. 2002-171287, any obstacle, such as a tall building, between the base station and a communication terminal may interfere with radio reception by the communication terminal, and in such a poor reception state a high transmission output is maintained. This may cause the transmission power amplifier to overheat.